1. Field of the Invention
The present invention relates to a gyro sensor (or yaw rate sensor) wherein a physical quantity such as a yaw added to the sensor is detected based on a detecting signal transmitted from a vibrator when the vibrator is vibrated in self-excitation in response to a driving signal transmitted from the vibrator.
2. Description of Related Art
A conventional gyro sensor is, for example, installed in a vehicle to detect a yaw rate indicating an angular velocity of the vehicle. In this sensor, a feedback loop is formed by a vibrator and a driving circuit to self-excite the vibrator at its natural frequency, and a rate of yaw added to the vibrator is detected in a yaw detecting circuit. As a type of gyro sensor, a micro gyro sensor of an electrostatic driving and capacitance detecting type has been disclosed in Published Japanese Patent Second Publication No. 2084567.
In a vibrator of this micro sensor, a fixed driving element adjacent to a movable element via a space is disposed as a driving sensor element, and a pair of fixed detecting elements adjacent to the movable element via a space are disposed as a pair of detecting sensor elements on both right and left sides of the vibrator. When a driving signal transmitted from a driving circuit of the sensor is added to the driving element, the movable element is vibrated in self-excitation along a driving direction. When a yaw is added to the sensor, the movable element is further vibrated along a detecting direction perpendicular to the driving direction, and a capacitance between each detecting element and the movable element is oscillated. Each detecting element outputs a detecting signal indicating the oscillating capacitance. In a yaw detecting circuit of the sensor, each detecting signal is changed to a voltage signal in a charge amplifier. A differential output is obtained from the voltage signals in a differential amplifier. This output passes through a synchronous phase detecting circuit, a low pass filter and a zero-point and temperature depending sensitivity adjusting circuit, and a sensor output indicating a rate of the raw is finally outputted.
In this micro sensor, the driving signal is obtained from a vibration detecting signal outputted from the driving element while using a power source normally set at 5 voltages and disposed outside the sensor. However, even though the driving signal set at 5V is supplied to the vibrator to continue self-excitation of the vibrator, the vibrator is not continuously self-excited in response to the driving signal of 5V. Therefore, a sensor output correctly indicating a yaw rate cannot be sufficiently obtained. To reliably continue the self-excitation of the vibrator, it is required to boost a voltage of the driving signal to a high voltage not less than 5V.
Further, in a conventional gyro sensor other than the micro gyro sensor, when the supply of electric power to the sensor is started, self-excitation of the vibrator is started in response to noises generated in a power source of 5V. In contrast, in the micro sensor, self-excitation of the vibrator cannot be started in response to noises generated in a power source of 5V. Therefore, to reliably start self-excitation of the vibrator in the micro sensor, it is required to boost 5V of a signal obtained from the power source to a high voltage and to apply a signal of the high voltage to the vibrator.
Especially, in the micro sensor, self-excitation of the vibrator is continued at its natural frequency in response to a driving signal when a frequency of the driving signal matches with the natural frequency, and a clock signal outputted from the self-excited vibrator is also used as a boosting clock signal required to generate the driving signal of a high voltage by boosting 5V of a signal obtained from a power source to the high voltage. Therefore, the driving signal of the high voltage cannot be obtained until the self-excitation of the vibrator starts is started to generate the clock signal in a feedback loop of a sensor circuit, and the self-excitation of the vibrator cannot be rapidly started unless the driving signal of the high voltage is not rapidly obtained.